Abstract
The DC poling field dependence of piezoelectricity was investigated to evaluate the mechanism of domain alignment in lead-free ceramics of the form of (1-x)(Na,K,Li,Ba)(Nb0.9Ta0.1)O3–xSrZrO3 (x=0–0.07) by comparison with those of Pb(Zr,Ti)O3 (PZT), PbTiO3 (PT), and BaTiO3 (BT) ceramics. Poling was conducted at 150 °C for 30 min while varying the poling field (E) between ±4.0 kV/mm. By increasing x from 0 to 0.07, the relative dielectric constant (εr), electromechanical coupling factor in the planar mode (kp), frequency constant in the kp mode (fcp), and piezoelectric strain constant (d33) vs E plots showed domain clamping at a specific E. E was the coercive field estimated from the DC poling field dependence. The changes in εr, kp, fcp, and d33 with E became smaller at x=0.06–0.07, because of the proximity to the paraelectric phase. The ceramics with x=0, such as PT or BT ceramics, show similar εr, kp, fcp, and d33 vs E plots. The maximum kp (48%) and d33 (307 pC/N) were obtained for x=0.05 with the lowest fcp of 2964 Hz·m, as shown in the εr, kp, fcp, and d33 vs E plots for ceramics such as tetragonal hard PZT ceramics. Since domain alignment in the ceramics was accompanied by the deformation of crystals as a result of applying the poling field, it was clarified that the lead-free ceramics must have a high kp and high d33 to realize a low fcp, which corresponds to a low Young's modulus. In addition, the optimal ceramic composition was obtained in the typical domain-clamping state from the poling field dependence. Furthermore, it was found that a higher kp was realized at a larger remnant polarization and a smaller coercive field in a symmetrical hysteresis loop of polarization vs electric field (P–E hysteresis loop), because of the easy alignment of ferroelectric domains by applying a poling field. The results of the poling field dependence were also supported by results of expansion strain measurement.
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